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Thanks for the responses gents, sorry to keep you waiting, was sending off my first attempt in Vermont for a different machine.

In, out and under and slightly smaller size, never used postimages b4, shiny stuff underneath more sticky than oily, not thick at all.

Thats not oil on the crown topside, just melted aluminium. Yes, likely 45 year old seals, and was actually tires until a few years ago, black and round:)  I'll peep the crank phase when I work on it again, R head loose but not off so pretty easy.  Unless it moved, It should be where the built it.  Seals are always a concern but it exhibited no unusual behavior.

Was running totally well and good until I thought I heard a funny clank(likely the ring down the pipe) and lost a bit of power, was somewhat concerned but my attention was on keeping it between the trees. Generally cover the clutch anyway when going quickly.

There were definately things bashing around, namely the rings, did some nice work on the head as well:)  Blame is squarely on me, probably done 4-6k on it. 

Very suprised at the lack of damage and that it ran reasonably well for probably 20 miles after the clank. 1 kick bike that morning after sitting probably 7 months, had 50/50 aviation fuel and no ethanol in it with a spash of castor for storage.  Tank has not been treated ever and has surface rust so it stays full or empty and fogged. 

After the incident and with the piston as you see, was still just a 4 kick bike.  Knew something was bad when I found the plug closed up.

Pin was about as tight as they are new, didnt even have to get behind it with a hammer, nice parts Yamaha:)

I seem to remember seeing that there was a rash of counterfeit NGKs showing up about everywhere and, of course, they were very unreliable.  First time I've heard of the HS not being produced, there are substitutes as mentioned above and even others I would assume.

I generally only change em if they are causing a problem after seeing how alot of very, very old plugs run fine if everything else is in good condition.  I've found that its rare that the spark plug is the only issue, its more likely to be old coils, bad wires or a bad cap.  I will agree that they are an easy change to see if it helps and do generally carry them on rides.

Every time I think I'm gonna sell one, I go for one last ride and think, well, maybe I do have space for this one....

My condolances for your loss, a difficult time, I'm sure:)

Back to the reversed slides experience, I like to forget to hook up the choke crossover:)

Maybe make sure your floats are actually floating while you are in there as well.  The float shutoff valves that are all metal never seem to cause issues for me but maybe I'm just lucky.

If your floats arent floating, the local small engine sling blade swears by SealAll.  I've used it a few times and dont remember having to go back and fix anything.  Its like a thick, old school model glue with a good bit of solvents in it, I keep it in a jar after the tube is opened.  Make sure the floats are dry inside first. 

Blew up the left side while seemingly running normally, blown thru the first ring and wasnt running too bad like this, hopefully the pic shows up.  Quickhand touch test on fins was pretty close during the ride, L was maybe slightly hotter, maybe.

I'm guessing airleak or timing but timing was late if anything on a quick check and I didnt find anything obvious on teardown. Rt cylinder 100% fine, piston dark but no signifcant carbon.  Tiny hole clogged but thats about it.

12k mi or so, never been apart, OEM 64mm piston, think we cleaned the carbs at the gap a few years back when Paul had just gotten his bike and we did the SV coil thing.  Originally had diode problems and siezed calipers, otherwise actually started and ran pretty decent, was stored semi well, PO bought for the wheels and never moved them to a 350.

Likely ran it with fuel off on the sidestand before draining tank but did find no gas in left bowl and was suprised to find a good bit in the right, did them on different days and didnt think I was going to pour a bowl of gas on my hands but did.  Tested petcock and float operation, all normal. No screen in the petcock so found some powdered tank rust in the bowls.

Foamie Uni likely going in the stock airbox, thinking 125/130 mains with 30 pilot. 115/25 was in it but airscrew liked 1/2 out only. Hopefully going to leakdown test, definately going through the timing well, maybe I can find my light instead of just setting it slightly late static.

Thanks for reading and thanks in advance for anything I could be missing, hope u like the pic, seemed like a pretty good one:)  Cyl actually not destroyed, dont know how that ring got out cleanly.

Patience and the above mixtures will work, eventually.  Making a piece of wood or hard plastic helps when its time for the beatings. 

Did a Lambretta last year that took about 3 weeks of soaking with all sorts of stuff, think I even heated some up.

All came apart eventually, heated the piston submerged in diesel outside for an hour on a hotplate in an old saucepan or so and got the rings out. Light hone and it actually runs with all the original parts, it was a see if it works thing.  After all the effort to get everything apart, I figured it'll be easy the second time:)

Did check the flashpoint, monitored temperatures and probably had it hanging so it wouldnt rest on the bottom.

There is a "ride out" in Miami on MLK weekend that typically has pretty good attendance.  Multiple reasonings/justifications for it but generally alot of shenanigans.  Worth a peep if you like that kind of thing, I keep thinking I should attend just to add to the critical mass of bikers.  Slow ass, legal Kymco aint gonna draw no trouble for sure:)

IIRC, a few years back, it was discovered that copycat/fake NGK iridiums were being being sold.  I presume, primarily online but there was a huge bad batch of vespa clutch bushings that were coming from reputable suppliers 5 or 6 years ago.

I find it incredible that this would be the problem here but it does follow the rule of "what did you F with last" method of troubleshooting:)

Glad to hear there was no physical damage to the rider, bikes can be fixed.  In fact, OP will likely just make it faster

The problem with the old ones is that they seem to like to slowly fail and make you think you have a carburetion, fuel delivery or points problem.  Which, come to think of it, one side of the 400 is running not quite as good as the other even though the plugs look identical....hmm

Jump it straight to a battery to see if it makes real noise without any of the circuitry.  I'd think a new horn should be loud as F.  I've got quite a few old ones lying around here and they have to be pretty trashed before theyre not loud.  Air horns typ need a relay iirc.

If youd rather, or if its easier, press the button and measure V at the horn, compare to what the battery reads, could be losing juice on the way.  Straight to the battery with a good connection eliminates all guessing about how loud it should be.

Ive taken a wind out of flatslides before to lighten the throttle so I see where you are coming from, but I love the pump on a street bike:)

25-30:1 guy here with jaso or tc oil in the aircooled gear.  Theres an article around somewhere with a test for maximum power from back in the day and IIRC it may have been 20 or 16 to 1.  Also, iirc, i think they still ran even down to 6 or 8 to 1.  Oil is easy to buy, cylinders, cranks and pistons, not always.

MX bikes run in tight woods tend to load up a bit as well, totally normal, clean em out on the hill climbs or in the fields.  Seeminglu less of a problem on bikes meant for trail riding.

I've done a fair amount of experimentation on an old weedeater and a 50cc razz with injection and it seems that any oil will work, even cooking oil and used motor oil.  They dont all work well and tend to junk things up but they dont sieze immediately, usually theres a noticible decrease in power and increase in heat.  If it feels like its running out of gas but shouldnt be, pull in the clutch:)

More of any oil, appropriately jetted, does not seem to increase smoke, if anything, less oil seems to smoke more.  Maybe less lube, more friction, more heat, more smoking oil?  Someone probably knows:)

Just my .02, plenty of ppl round here that have experimented at a higher level and with real consequences

Have indeed had to clean these out of carbs before, dont know if they were unis, they almost seemed like upolstery from what I remember.

Redid one of these that was built from a kit a few years back.  Things that I remember were important were chain alignment and tension, the kits come with a fixed tensioner and since the gearing is clamped/sandwiched to the left side spokes, its tricky to get it to run true and circular.  In the kit I ran lots of oil and tuned accordingly 20:1 iirc.

Brakes are important, its not much more weight or speed but it is more of both, make sure theyre working good just incase that kill switch doesnt work.  The kits have a hand clutch thats used for starting, has a latch to hold in as well in case you are just pedalling.

I got mine going about 35 by the time I was done, no big mods just getting everything as good as it could be.  Rubber mounted engine was smooth to about 20, terrible after that but plenty of go.  Big ass beach cruiser, truckin along:)

I found it fun but it was a heavy ass bicycle and not a very good scooter and I didnt need it at all so it went to someone more transportationally challenged.  Yours should be fun as well and maybe alot better if it runs smooth at the top end, Id guess that the motor is a good bit lighter so it might be more bicycle like.  20 without pedalling and that sweet sound is pretty good, especially if you have favorable laws.

Just so happens I just picked up one of these in similar condition for a YSR swap and have been meaning to list up the rest of the machine here.  Think mine is 74 as well, greenish gold?  Goldish green?  Cant remember, we took it apart after we got it sparking and I left most of it in my buddys shop.  Thanks for the motivational thread, GL with the build, I thought about it but I do like the YSR.

I should have whatever you need and many of the parts are in good condition, I'll make sure my email is good here and I may be able to get pics et. al. soon, parts coming for a pair of Vespas saturday and the YSR is next in line.  Low, low prices for the 2SW peeps, I rarely get around to selling on Ebay, and things just go in boxes.